130 



ME. G. W. C. KAYE ON THE 



platinum respectively. The percentage transmitted, of the radiation from platinum, 

 is indicated for various thicknesses of screen. 



It ought to be mentioned that the silver anticathocle soon amalgamated with the 

 mercury vapour from the pump, and its surface thus became coated with an alloy of 

 the probable composition Ag.Hg. 



Aluminium Screens. Consider fig. 3. As the thickness of aluminium screen is 

 increased, lead and silver increase their radiation values, and take places warranted 

 by their atomic weights. Thus the softest rays due to a lead or silver radiator are 

 more penetrating to aluminium than the softest rays from platinum. 



100 



THICKHfSS a, AL 



Fig. 3. Aluminium screen, 28,000 volts. 



The metals of the iron group copper, nickel, and iron rapidly lose, with thicker 

 screens, their initially high radiation values. Together with aluminium they afterwards 

 show weak maxima at a thickness of about 0'07 cm. of screen. Thus for the range 

 0'03 to 0'07 cm. the rays from these metals are more penetrating to aluminium than 

 are the rays from platinum, and we thus have a region over which selective 

 transmission is manifested, 



It will be noticed that for screens thicker than about 2 mms. an alteration in the 

 thickness produces very little change in the relative amounts of radiation from the 



